1. Field of the Invention
The present invention relates to an oil-well installation fitted with an electric pump at the well bottom.
2. Description of Related Art
In some oil wells, the natural flow of the hydrocarbons from the bottom to the surface proves insufficient to allow or maintain commercial production. This is due either to the high viscosity of the hydrocarbons or to an excessively low natural pressure at the bottom of the well, or else to a combination of the two. In order to allow the well to enter production on a commercial scale, an assistance system or an activation system for the well may be used. For example, a pump may be provided at the lower end of a production tube located in the well. This pump can be operated by an electric motor immersed at the bottom of the well, which is powered by a cable arranged in the annular space between the tubing and the casing of the well.
When making an inventory survey of the sources of breakdowns in electric pumps immersed at the well bottom which require withdrawal of a tubing, the following are found in order:
electrical short-circuits (approximately 80%), PA1 mechanical wear, PA1 mechanical failure. PA1 Working the well necessarily generates pressure and temperature variations, due to stoppages and to changes in working conditions, in the regions where the pumping equipment is located, thereby creating mechanical stress cycles in the constituent materials. PA1 The pressure variations also cause repeated gas migrations within the insulators, which tend to degrade their structure and their performance. PA1 The presence of hydrocarbons, aromatic compounds, acid gases and the like causes various chemical attacks on the various mechanical or electrical insulation barriers, thus contributing to their degradation over time. PA1 The variations (in particular stopping and starting the motor) in electric current flowing through the electrical conductors generate, by the Joule effect, significant temperature variations which accelerate ageing of the electrical insulators. PA1 The high voltages mentioned above generate strong stress fields on all the insulators.
Of course, some short-circuits arise when first starting up and result from a fault during installation which requires a great deal of care and knowhow.
However, most short-circuits take place during use and result from normal progressive degradation of the electrical insulation barriers, since the pumps are immersed in the effluent extracted from the oil-bearing rock.
The power range and the installation depth require high voltages, which may be up to 1000 to 3000 volts, in order to minimize losses in the cables. However, these high voltages make the installations vulnerable.
Damage to solid insulators results from phenomena which are easy to understand:
The second cost factor for using electric pumps immersed at the bottom of a well is that, in order to repair a breakdown, all the electrical cable and the tubing to which the bottom unit is coupled must be raised back up. Current bottom-well electric pumps actually constitute compact tubular blocks assembled at the surface before they are lowered into the well. These systems must consequently be raised integrally with the tubing when carrying out maintenance. This operation requires service equipment which is expensive to mobilize, especially on sites with difficult access (those which are isolated, at sea, underwater or urban sites). The waiting time and operating time also generate significant production losses. This is true to the extent that these systems cannot be considered in the most difficult cases.
All these factors dramatically affect the cost of this pumping method and even economically rule out the development of certain marginal oil fields.
A sucker-rod pumping installation consists of a positive displacement pump which is installed in the tubing and has its piston moved in translation from the surface by means of steel or glass-fibre rods. At the surface, the movement is imparted to the rod line by a rocker structure driven by a rotary electric motor or else a hydraulic jack.
The intrinsic weight, the inertia, the friction and the mechanical fatigue of the rods limit the pumping capacity and performance of these systems. They are ill-suited to flowing wells, for which bottom safety components are required, to deep wells or to high flow rates (greater than 200 m.sup.3 /d of liquid).
The monobloc design of current bottom electric pumps is beneficial in the case of wells where the handling operation is easy and inexpensive, which is the case for drinking-water wells, non-flowing on-shore or shallow wells. It is not suited to current and future oil wells. These are increasingly deep, inaccessible, dangerous (because blowout often occurs), and are fitted with complex equipment which is difficult to install. It is becoming desirable to concentrate the unavoidable weaknesses such as mechanical wear in an independent module which would be more lightweight and less expensive to raise up and replace, with a cable or a winch for example.
Given that most causes of breakdown are due to the fact that the electrical part is immersed in a hostile environment, it would be desirable to group the whole of the electrical part in an enclosure shielded from any attack which could lead to electrical breakdowns. This would leave in contact with the effluents only the "mechanical" components which cannot cause or suffer electrical short-circuits and can be raised up independently and replaced for the purpose of maintenance, following mechanical wear, or for flexibility in order to adapt, for example, to a change in the effluents from the well or the working conditions.